Among thermal joining processes in modern manufacturing technology, autogenous fusion welding has traditionally been ubiquitous within the metal construction industry. Although the roots of this process can be traced back to primitive welding of noble metals and eutectic alloys for the jewelry and armor of prehistoric times, until the development of the oxyfuel torch by Bunsen and electric carbon welding by Bernados in the previous century, fusion welding was not widely accepted on the factory floor. Besides classical methods such as Gas Tungsten Arc Welding (GTAW), new emerging techniques including Plasma Arc (PAW), Laser Beam (LBW) and Electron Beam Welding (EBW) are currently penetrating the production industry. These modern methods, combined with automated mechanized and robotic torch motion systems, enable closer control of the weld bead geometry, the material structure and properties, and the thermal stress or distortion effects of the weld, thus contributing to an enhanced joint quality and productivity of welding operations.
In contemporary industrial practice, the process conditions, such as the torch power and motion, are selected according to empirical recommendations in order to obtain the desired characteristics of the final weld. To handle the welding transients such as the material and torch parameter uncertainty and process disturbances, sophisticated in-process control systems have been proposed which employ measurement and feedback of important lumped weld variables to modulate the torch intensity and speed in real-time. In these systems, the weld features or characteristics result from a single, localized, sequentially moving torch or weld head. Thus, the steep temperature distribution and high cooling rates accompanying a well-penetrated weld. In hardenable high-strength steels, for example, these steep distributions may result in a martensitic structure and residual stresses which render the material practically unweldable by the concentrated heat source from a practical stand point.
Multiple torch schemes with independently modulated heat inputs have been tested to spread out the heat input distribution. Predefined weaving patterns of the torch motion have also been proposed. The multi-torch implementation, however, is rather impractical and costly, and standard mechanized weaving provides only limited additional flexibility in the welding conditions.